An important question in Huntington's disease (HD) pathogenesis is whether mutant huntingtin (Ht) effects striatal cell death directly by perturbing striatal cell fL motion or whether it alters areas outside the striatum (notably cortex) that influence the striatum, leading indirectly to striatal cell death. This could be studied by examining the effect of a 100% mutant cortex on a 100% wild-type (VVT) striatum, and vice versa. We propose to develop a novel model system, creating in oculo co-transplants of mixed genotypic pairings, using embryonic HD mutant mouse and WT tissue, of cortex and striatum. Our studies will develop the in oculo method to test the competing hypotheses: 1) that a cortical action of mutant Ht drives striatal injury, even in the absence of the HD mutation in striatum, 2) that striatal injury can occur in a mutant striatum, even in the presence of an input from a WT cortex or in the absence of any cortical input or 3) that striatal injury is driven by a combination of a cortical and striatal action of mutant Ht. In Aim 1 we will create in oculo co-transplants using embryonic HD mutant cortical tissue and embryonic WT striatal tissue and look for signs of striatal cell injury as compared to co-transplants of WT cortex and WT striatum. If the hypothesis is correct, WT striatum wilt show increased signs of injury when co-transplanted with a HD mutant cortex. In Aim 2 we will create in oculo co transplants using embryonic WT cortical tissue and embryonic HD mutant striatal tissue and look for signs of striatal injury as compared to WT controls. If striatal injury depends on cortical input, but not mutant input per se, a HD mutant striatum will show signs of injury when co-transplanted with a WT cortex. Additionally, we will create single transplants of embryonic mutant striatum and look for signs of striatal injury. If striatal cell death is entirely cell autonomous, even in the absence of cortical input, striatal injury will occur in a HD mutant striatum, in Aim 3 we will create co-transplants using embryonic HD mutant cortex and embryonic HD mutant striatum and look for signs of striatal injury as compared to controls. This hypothesis predicts that striatal injury will only occur when an HD mutant cortex is co-transplanted with an HD mutant striatum or that it will be more severe than if only cortex or striatum is mutant. These studies will validate the in oculo technique as an effective HD model system, which might be useful in further testing hypotheses of mutant Ht mechanism of action and in testing possible drug therapies.